P2636R0
References to ranges should always be viewable

1. Revision History

1.1. Revision 0 - 2022-09-14

• Initial version.

2. Introduction

When dealing with views, two concepts are important: view ("ranges that are views themselves") and viewable_range ("ranges that can be turned into views"). Initially, the requirements for the first were very strict, encompassing only non-owning, semi-regular types. The second concept encompassed "views and any lvalue-references to ranges".

Over time, both concepts were relaxed considerably. Views can now be move-only (P1456) and owning (P2415), rvalues of non-views are now viewable––in fact most ranges people encounter are now viewable. However, in one particular case, the concepts are now stricter than before: lvalue-references to certain ranges, move-only views, are not viewable. This means that while almost all non-view ranges can be easily composed with view adaptors, certain views now do not qualify for this.

This issue was briefly discussed on the LWG reflector in 2020 where another member highlighted the wide-ranging impact of this change:

I’m concerned about having components that work with ranges but not views, given that views are supposed to be just ranges with additional properties.

A possible "fix" was mentioned in that discussion:

I can envision an alternate design in which views:all(E) does admit move-only lvalue views [...] I think that design change needs LEWG approval [...]

But apparently there was no support for this change or no will to pursue it further. This paper picks up that discussion and argues why such a change would indeed make a lot of sense.

2.1. Minimal problem description

void foobar(ranges::forward_range auto && r)
{
    auto v = r | views::take(3);
}

In the original C++20 standard and in multiple shipped compiler versions, the above code was valid for all forward_ranges. Since P2415, the function template is now underconstrained and will fail for certain inputs.

3. Detailled examples

To illustrate the different behaviours, I have chosen to provide examples that start out with (1) a copyable view, (2) a copyable non-view range, (3) a move-only non-view range, and (4) a move-only view.

As example types for non-copyable ranges, I use Generator and GeneratorView. Generator could refer to a user-provided type that generates objects upon iteration (e.g. records of data from a stream); it is a move-only single-pass input-range.

The type GeneratorView is the same as Generator except that it also inherits from the empty base class ranges::view_base. This has no influence on the semantics of the type other than it begin treated as a view by current standard libraries. While we might suggest to users that such ranges should always be implemented as views, we cannot assume that this will always be the case (there are ranges that are neither containers nor views!).

3.1. Views from lvalue reference

string_view s{"foobar"};
auto v = s | views::take(3);

string s{"foobar"};
auto v = s | views::take(3);

Generator g{};
auto v = g | views::take(3);

GeneratorView g{};
auto v = g | views::take(3);

Before P1456 and P2415, all of these snippets would have been valid. Currently, (4) is not, because lvalue references to move-only views are not viewable.

This also means that code is rejected by current compilers that was accepted by previous versions of the same compiler (GCC, MSVC). While this is true for other C++20 features, the particular changes to the view concepts were discussed mainly as relaxing constraints and not as introducing new ones.

3.2. View composition (starting from lvalue reference)

string_view s{"foobar"};
auto v1 = s  | views::take(3);
auto v2 = v1 | views::transform(/**/);

string s{"foobar"};
auto v1 = s  | views::take(3);
auto v2 = v1 | views::transform(/**/);

Generator g{};
auto v1 = g  | views::take(3);
auto v2 = v1 | views::transform(/**/);

GeneratorView g{};
auto v1 = g  | views::take(3);
auto v2 = v1 | views::transform(/**/);

(4) is still invalid; the rest are valid.

3.3. Views from rvalues

auto v = string_view{"foobar"} | views::take(3);

auto v = string{"foobar"} | views::take(3);

auto v = Generator{} | views::take(3);

auto v = GeneratorView{} | views::take(3);

All of these snippets are currently valid code. In the original ranges proposal, only (1) would have been valid, because the others are rvalues of non-views; (4) would not have been regarded as a view, because it is move-only.

3.4. View composition (starting from rvalue)

auto v1 = string_view{"foobar"} | views::take(3);
auto v2 = v1 | views::transform(/**/);

auto v1 = string{"foobar"} | views::take(3);
auto v2 = v1 | views::transform(/**/);

auto v1 = Generator{} | views::take(3);
auto v2 = v1 | views::transform(/**/);

auto v1 = GeneratorView{} | views::take(3);
auto v2 = v1 | views::transform(/**/);

(2)-(4) are not valid, because v1 in the respective snippet is move-only. Note, how the valid snippets in this example are different from those created from lvalues. If the | views::transform(/**/) had been appended to the first line and no v2 created, it would have worked. This means that––contrary to what many previous ranges guides suggest––it makes an important difference now whether you compose the view incrementally or in one line. And it appears that "creating in one line" needs to be recommended.

3.5. Generic code

Returning to the problem of generic code, assume the following:

(definitions for to_upper and to_lower omitted)

void print_up_low(ranges::forward_range auto && rng)
{
    for (char c : rng | views::transform(to_upper))
        cout << c;
    cout << '\n';
    for (char c : rng | views::transform(to_lower))
        cout << c;
}

print_up_low(string_view{"foobar"});
print_up_low(string_view{"foobar"} | views::take(3));

print_up_low(string{"foobar"});
print_up_low(string{"foobar"} | views::take(3));

string_view s{"foobar"};
print_up_low(s);
print_up_low(s | views::take(3));

string s{"foobar"};
print_up_low(s);
print_up_low(s | views::take(3));

These examples show that it is easy to run into problems with "move-only views", even when just using simple adaptors on types likes strings and string_views. Is the behaviour shown above intended? Do we expect developers to understand why 1 of the 8 the calls to print_up_low() is invalid and why?

Curiously, it is now the example of "creating a view in one line" that is invalid, while previous examples suggested that exactly this usage pattern increases the likelihood of the code being valid.

4. Proposal

This papers suggests making all previously shown invalid snippets valid by changing the viewable_range concept and the definition of the views::all adaptor.

26.4.5 Other range refinements [range.refinements]

Change the definition of viewable_range to:

template<class T>
  concept viewable_­range =
    range<T> &&
    ((view<remove_cvref_t<T>> && constructible_­from<remove_cvref_t<T>, T>) ||

(!view<remove_cvref_t<T>> && (is_lvalue_reference_v<T> || (movable<remove_reference_t<T>> && !is-initializer-list<T>))));

is_lvalue_reference_v<T> || (movable<remove_reference_t<T>> && !is-initializer-list<T>));

[26.7.6.1 General [range.all.general]]

Change the definition of views:all to

• (2.1) decay-copy(E) if that expression is well-formed and the decayed type of E models view.
• (2.2) Otherwise, ref_­view{E} if that expression is well-formed.
• (2.3) Otherwise, owning_­view{E}.

These changes are breaking insofar as all concept changes are always potentially breaking. However, this proposal very clearly relaxes the constraints on viewable_range so the scope is broadened and in practice more code will work than before and not less. In fact, this will unbreak code that was valid in earlier iterations of the standard (and is valid in released versions of GCC and MSVC) but that has been broken in the meantime.

5. "Workarounds"

For the previously shown invalid snippets, it is possible to work around the problem by any or either of the following methods:

1. move-ing the initial range into the pipe (resulting in an decay_copy or owning_view); or

2. manually wrapping the initial range in ref_view

Since most users never directly interact with views::all, ref_view and owning_view, I assume that it will be very hard to explain where (and which of) these workarounds are necessary. Furthermore, this creates a burden on generic code that will either need to if constexpr on ranges passed to it by reference or blindly wrap them in ref_view to make them viewable.

6. Lifetime

Previous mailing list comments suggested that the changes proposed here would create issues with view lifetime. The following two examples explore some of those implications.

6.1. Returning views on local variables

string_view s{"foobar"};
return s  | views::take(3);

string s{"foobar"};
return s | views::take(3);

Generator g{};
return g | views::take(3);

GeneratorView g{};
return views::take(3);

Returning a view created from a local variable can create a dangling reference. This has always been the case and is inherent to ranges::ref_view (and independent of the underlying range). While it could be argued that preventing it for some ranges is beneficial, it is unclear why this should only be the case for move-only views and not other ranges; especially since example (2) is likely to appear more often than example (4) and the range in (3) is semantically identical to the range in (4).

On a sidenote: Now that we have move-only owning views, one could make an argument for making all uses of ranges::ref_view explicit (removing the option from views::all and recommending that users move into a view composition by default or explicitly opt into having a reference if desired). However, such a change would be quite disruptive, so pursuing this argument at this point seems futile.

6.2. Returning views on in-parameters

auto foo(string_view & s)
{
  auto v1 = s | views::take(3);
  return v1 | views::transform(/**/);
}

auto foo(string & s)
{
  auto v1 = s | views::take(3);
  return v1 | views::transform(/**/);
}

auto foo(Generator & g)
{
  auto v1 = g | views::take(3);
  return v1 | views::transform(/**/);
}

auto foo(GeneratorView & g)
{
  auto v1 = g | views::take(3);
  return v1 | views::transform(/**/);
}

Example (4) would become valid code through the changes proposed here. Contrary to what one may assume, the example will not return a dangling reference: while s is move-only, v1 will be a ref_view to s and thus copyable; in the return statement, it is copied into the returned view, so the composition does not dangle.

7. Summary

Treating move-only views as fundamentally different from copyable views when evaluating view-ability, creates yet another source of user error and frustration. I agree with the changes made to views, but we now have problems explaining what a view even is (P2415); having two sets of views with different usage patterns definitely won’t help. This does not only affect exotic, user-defined types, but can become obvious even in toy examples created with strings.

The difference becomes even more pronounced with standalone move-only ranges that can be implemented as views but where we cannot assume that they always will be. For those types, it is unclear what benefit opting into the view concept provides––if that only makes the type opt out of easy view composition.

Further complications arise in generic code, which could previously treat all views as ranges and now needs special case handling.

While it is true that all direct and indirect uses of ranges::ref_view entail the possibility of dangling references, it is unclear why this is considered more dangerous for move-only views than other ranges. I hope to have alleviated some concerns in this regard and shown that the proposed increase in consistency and usability provides a clear benefit.

8. References

• Short 2020 discussion in LWG: https://lists.isocpp.org/lib/2020/09/17353.php

• Short 2022 discussion in LWG: https://lists.isocpp.org/lib/2022/08/23438.php

• Brief mention in minutes: https://wiki.edg.com/bin/view/Wg21telecons2021/LWG-issues-20210115

• GCC issue with other examples: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106669